1. Field of the Invention
The present invention relates in general to testing normally closed check valves at the outlet of a passive injection system, and more specifically to testing check valves connecting accumulator tanks to a reactor cooling system in a pressurized light water nuclear power system.
2. Description of the Related Art
Check valves are commonly used in many types of applications to prevent flow in a particular direction except when the pressure upstream of the check valve exceeds the pressure downstream of the check valve. One such application is in a pressurized system to maintain pressure at a certain level. In such applications, a check valve may be connected at the outlet of a tank which is pressurized to a predetermined pressure. The check valve is opened only if the pressure in the system connected downstream of the check valve falls below the predetermined safe pressure. This type of application is termed a passive injection system. Included in the types of installations which use passive injection systems are pressurized light water nuclear power systems.
A simplified diagram of a pressurized light water nuclear power system is illustrated in FIG. 1. In FIG. 1, a reactor vessel 10 generates hot pressurized water, typically at a pressure of approximately 2250 psig and a temperature of approximately 600.degree. F., which exits the reactor vessel 10 in a pipe identified as the hot leg 15. The pressurized hot water is routed through a steam generator 20 to produce steam which may then be used to generate electricity. The water which exits the steam generator 20 via a crossover leg 25 is at a reduced temperature and passes through a pump 30 to be returned to the reactor vessel 10 via a cold leg 35. All of the above described portions of the reactor cooling system (RCS) are enclosed by a containment boundary 40.
While only one loop of hot, cold and crossover legs 15, 35 and 25, respectively, is illustrated in FIG. 1, conventional pressurized light water nuclear power systems have two (2) to four (4) such loops. All of the loops include a steam generator 20 and pump 30, but connections to emergency core cooling systems (ECCS) 45 may vary from loop to loop. One or more of the loops will also have connections to a pressurizer 50 and a chemical and volume control system (CVCS) 55 which together control the pressure in the RCS. The pressurizer 50 reduces pressure in the RCS by injecting cold water via a spray head 60 and increases pressure by heating the water using a heater 65. Volume changes caused by the changes in pressure are corrected by the CVCS 55.
Nuclear power systems are built with multiple safeguards. One of the types of safeguards are the emergency core cooling systems (ECCS) 45. As illustrated in FIG. 1, the ECCS 45 lie partially inside and partially outside the containment 40, receive hot water from the hot leg 15 and supply water to the cold leg 35. There may be several different types of systems included in the ECCS 45, each operating at different times or with different objectives. For exampe, the material supplied by an emergency core cooling system may be just water at an unregulated temperature to maintain pressure, water at a carefully controlled temperature, water mixed with boron to control the reaction in the nuclear reactor, or other materials mixed with water or another liquid. The sources of water may include large unpressurized reservoirs such as a refueling water storage tank (RWST) 100 (FIG. 2) containing 350,000 to 500,000 gallons and pressurized accumulator tanks 105 holding, e.g., 1350 cubic feet of water and nitrogen pressurized to between 500 and 700 psig.
The ECCS 45 include high pressure systems which are used to help control the reactor early in a loss of coolant accident (LOCA) and typically include a connection to a boron injection tank 110, so that boron can be added to water injected by safety injection pmps 115 in the high pressure system. Low pressure systems are included in the ECCS and are designed to supply larger volumes of water. Included in the lower pressure systems may be residual heat removal (RHR) systems which remove heat at lower temperatures from the reactor cooling system (RCS) using residual heat exchangers 120 and low pressure pumps 125.
Every two months to one year, nuclear power plants go through a cold shutdown process in order that fuel may be added to the reactor or other maintenance operations performed. The cold shutdown process involves venting steam from the steam generators 20 while stopping the nuclear reaction, i.e., stopping heat generation, so that temperature and pressure in the RCS can be gradually reduced. When the pressure is reduced to approximately 1000 psig, the emergency core cooling systems ECCS 45 are manually located out to prevent inappropriate automatic activation, thus preventing flow through the ECCS 45. At approximately 450 psig and a temperature of no greater than 350.degree. F., flow from from as least one of the hot legs 15 through the residual heat removal (RHR) system is started using the low pressure portion of the ECCS 45 to reduce the temperature below 200.degree. F. so that the pressure can be reduced as low as atmospheric pressure.
During the cold shutdown process, water is routed through the heat exchanger 120 (FIG. 2) in each RHRS from one of the hot legs 15 via a pipe 130 when two isolation valves 135 which are normally closed, as indicated by the dark shading in FIG. 2, are opened. At the same time, shut off valve 137 can be closed to prevent water being drawn from the RWST 100. The RHR pump 125 is turned on and water flows through check valves 140, 145 and 150. Check valve 155 prevents the water from flowing into the accumulator tank 105 and instead the water is pumped through check valve 160 to the RCS. Thus, any malfunction of check valves 140, 145, 150 or 160 will be detected during the cold shutdown process. However, there is no ordinary or normal operation of a nuclear power plant which tests the operation of check valve 155.
Section XI of the American Society of Mechanical Engineers (ASME) Code requires periodic inspections and testing of various components in nuclear power plants. Subsection IWV of Section XI addresses the testing of valves. Paragraph IWV-3520 requires that check valves be exercised at least every three months unless such operation is not practical during plant operation, which is true for the valves in the ECCS 45. All valves not tested every three month are required to be full stroke exercised during cold shutdowns (Subparagraph IWV-3522).
There are several different types of check valves. Conventional check valves include ball and swing-disk check valves. These valves can only be tested by adjusting the pressure upstream and downstream of the valve until they open. One type of check valve includes a "swing arm" on the exterior which permits mechanical actuation to test the valve. Subparagraph IWV-3522(b) permits testing by such a mechanical exerciser (swing arm).
There are several drawbacks to using a check valve having a swing arm. First, there is no way to be certain that the valve is moving when the swing arm is moved on the exterior of the valve. Second, the valve is not actually being tested in operation, i.e., due to a pressure differential upstream and downstream of the valve. Third, the axle turned by the swing arm provides an additional location for leaks to occur.
Due to the drawbacks of the wing arm type check valves, other alternatives are used for valves which are not tested by any ordinary operational procedure, including cold shutdowns. One alternative is to totally disassemble the check valve at each cold shutdown. Disassembling check valves can ensure that the check valves are still capable of movement, but like the mechanical actuation test, this does not ensure that the valves will operate at the required pressure differential. Another alternative is to simply request relief from ASME Code Section XI, Subsection IWV-3520. Although this alternative involves no testing whatsoever, it has been used by some nuclear power plants.
There is a procedure which tests the operation of check valve 155; however, this procedure is not used at each cold shutdown, but rather once every ten years or prior to initial operation and 3 times during the life of a 40 year power plant. This procedure involves a "full blowdown" of the accumulator tank 105 by opening isolation valve 165, typically a motorized gate valve, when the RCS pressure is approximately atmospheric pressure. The valves, piping and connections between the accumulator tank 105 and the cold leg 35 of the RCS are designed to withstand relatively few high pressure full blowdowns during the life of the nuclear power plant. Therefore, meeting the requirements of the ASME Code Section XI using high pressure full blowdowns would require redesign or additional analysis of the RCS and ECCS to determine whether maximum acceptable pressure and temperature differential transients resulting from operation of the accumulator tank 105 in the passive injector system are exceeded.